This invention relates to the field of acoustics, and more particularly to means for attenuating the acoustical component of the energy of a flowing gas. Devices for this purpose are known as mufflers or silencers, and numerous types thereof are known. A type of primary interest here is that in which the gas flows through a passage or tube coupled by a field of small apertures with one or several resonating chambers, each chamber being designed to attenuate acoustical energy in a predetermined frequency range.
From a constructional point of view, the most desirable structure of this sort is a simple inner tube having a field of sharp edged, round, regularly positioned perforations. The perforations can be punched while the metal is flat by a simple line of inexpensive punches, in one or two passes through the punch depending upon whether the perforations are to be in-line or staggered, the tube can thereafter be completed by rolling or forming and welding, and there is nothing to prevent the completed tube from sliding into any appropriately sized circular orifice in a muffler baffle or core, or to prevent appropriately sized circular members from being inserted into the tube, as any special demands of an application may require. The structure is rigid, durable, and resistant to high temperature and to plugging of the holes by particulate matter in the gas.
It has been found, however, that mufflers of this sort generate annoying whistling sounds, which in extreme cases may be of greater intensity than the broad-band sounds which are intended to be attenuated, and may lie within the design attenuation band.